DISULFIDE BONDS BETWEEN 2 ENVELOPE PROTEINS OF LACTATE DEHYDROGENASE-ELEVATING VIRUS ARE ESSENTIAL FOR VIRAL INFECTIVITY

Disulfide bonds were found to link the nonglycosylated envelope protei n VP-2/M (19 kDa), encoded by open reading frame 6, and the major enve lope glycoprotein VP-3 (25 to 42 kDa), encoded by open reading frame 5 , of lactate dehydrogenase elevating virus (LDV). The two proteins com igrated in a complex of 45 to 55 kDa when the virion proteins were ele ctrophoresed under nonreducing conditions but dissociated under reduci ng conditions. Furthermore, VP-2/M was quantitatively precipitated alo ng with VP-3 in this complex by three neutralizing monoclonal antibodi es to VP-3. The infectivity of LDV was rapidly and irreversibly lost d uring incubation with 5 to 10 mM dithiothreitol (>99% in 6 h at room t emperature), which is known to reduce disulfide bonds. LDV inactivatio n correlated with dissociation of VP-2/M and VP-3. The results suggest that disulfide bonds between VP-2/M and VP-3 are important for LDV in fectivity. Hydrophobic moment analyses of the predicted proteins sugge st that VP-2/M and VP-3 both possess three adjacent transmembrane segm ents and only very short ectodomains (10 and 32 amino acids, respectiv ely) with one and two cysteines, respectively. Inactivation of LDV by dithiothreitol and dissociation of the two envelope proteins were not associated with alterations in LDV's density or sedimentation coeffici ent.